Production of light-polarizing images



Jan. 4, 1949. H. P. 'HUSEK v 2,458,168

Y PRODUCTION 4 OF, LIGHT-POLARIZING IMAGES I Filed Aug. 27. 1945 IodineImage Moleculqry Oriefifed Currier Supporf.v

IN V EN TOR.

Mg/gm Patented Jan. 4, 1949 PRODUCTION "OF LIGHT-POLARIZING IIWAGESHelen P. Husek, Allston, Mass, assignor to.

Polaroid Corporation, Cambridge, Mass, a corporation of DelawareApplication August 27, 1945, Serial No. 612,876 Claims. (01. 95- 88)This invention relates to photography and more particularly hasreference to the processing of silver images to obtain iodine images ofdesired density and to products formed in such processing.

A method heretofore practiced for the formation of an iodine imageconverts asilver ferrocyanide and reacts the latter with a salt of apolyvalent metal in its higher valent form to provide a water-insolublemetallic .ferrocyanide which is converted to its correspondingwaterinsoluble hydroxide upon reaction with a base. The Water-insolublehydroxide thus produced is an oxidizing agent which when treated with aniodide in the presence of an acid will effect the release of iodine toprovide an image in iodine, the metallic hydroxide .being dissolvedduring this reaction.

An iodine image formed in this manner is apt to have a greater densitythan the silver image from which it was derived. It is possible to avoidthis undesirable over-densification of the iodine image by exposing theemulsion in which the silver image is formed to the end of providing asilver image of low density. For the amateur or novice, the practiceinvolving the formation of an image of low density is in generaldifficult to carry out, especially when depending on visual controls.

Objects of the invention reside in the provision of processes for theformation from silver images of iodine images of desired density; forthe control of the density of an iodine image in its formation from asilver image; and for the transformation of silver images inphotographic carrier materials to iodine images of selected density.

Other objects of the invention relate to the provision of methods fortreating silver images to form iodine images of desired density inphotographic carriers of hydrophilic material such as gelatin andespecially in transparent, linear polymeric plastics of the characterwhich may have their molecules oriented, as for example polyvinylalcohol; and also for the formation of dichroic iodine images ofpredetermined density in molecularly oriented materials whereby toprovide veetograph images, that is to say images the contrast of whichis a function of the direction of incident light; to employ processes ofthe character described wherein the photosensitive material is silverhalide; and to provide processes for the production in a photographiccarrier of an image containing an oxidizing agent which willcontrollably effect the release of iodine from an acid solution of aniodide.

Further objects of the invention relate to the provision in processes ofthe character described of thetreatmen-t of a silver image with amixture of two or more metallic salts, each capable of formingwater-insoluble ferrocyanides and each having comparable solubilityproducts, at least one of the salts being asalt of a polyvalent metal:

he element in higher valent form and at least another of said saltsbeing a salt of a univalent metallic element other than silver or a saltof a polyvalent metallic element in a lower valent form; and to provideprocesses for the DIOdLlCLlOIl of iodine images of controlled densityfrom silver images by the use of at least one salt of a univalentmetallic element other than silver orof a polyvalent metallic element ina lower valent form which is predeterminedly added to a treatingsolution for the silver image containing one or more salts of apolyvalent metallic element in higher valent form.

Still further objects of the invention reside in the product obtained bytreatment of a silver image inan image-bearing layer with at least onesalt of a polyvalent metallic element in higher valent form and at leastone salt of a univalent metallic element other than silver or a salt ofa polyvalent metallic element in lower valent form; and to a productcomprising a transparent, molecularly orient-ed, high polymer havingpredetermined portions thereof containing a relatively water-insolublecompound of a polyvalent material in higher Valent form and also arelatively water-insoluble compound incapable of releasing iodine froman iodide solution the lastnamedcompound being a compound of .apolyvalent material in lower valent form or a compound 'of a univalentmaterial.

The invention accordingly comprises the several steps and relation andorder of one or more of such steps with respect to each of the others,and the article possessing the features, properties and the relation ofelements, which are exemplified in the following detailed disclosure,and the scope of the application of which will be indicated in theclaims.

For a fuller understanding of the nature and objects of the invention,reference should 'be had to the following detailed description taken inconnection with the accompanying drawing which is a schematic View of aphotographic film having an iodine image formed therein and with whichthe invention may be practiced.

As indicated, the invention contemplates the formation of iodine imagesin image-bearing carrlers or layers of suitable material. By oneproformed may comprise any photographic carrier material. Materials usedfor image-bearing layers may be transparent and may be described ashydrophilic in that they have a strong affinity for water or absorb oradsorb water. At the same time the carrier materials used with theinvention may be termed relatively water-insoluble, reference being hadto hydrophilic carriers having solubilities which will prevent watersolutions used in processing from appreciably dissolving them during thetime required to carry out the photographic treatments described herein.Preferred examples of carrier materials comprise gelatin andtransparent, linear, polymeric plastics capable of having theirmolecules oriented as well as photographic printing papers.

Polymeric plastics, which are able to form a dichroic sorption complexwith iodine and which possess a relatively high tensile strength so asto perm-itthe orientation of the molecules thereof,

are especially useful in carrying out certain embodiments of theinvention. Materials falling within this description and possessingsufiicient strength to permit orientation of the molecules thereof areherein termed orientable plastics, all other materials being deemed tobe nonorientable.

Molecularly oriented plastics when stained with a dichroic stain such asiodine are rendered light-polarizing. However, if orientable materialsof this character are used in a substantially unoriented condition,iodine, while staining them will not render them light-polarizing. Thus,a

light-polarizing image may be formed by staining predetermined portionsof a molecularly oriented plastic sheet with iodine. Also alightpolarizing image may be obtained by the formation of an iodineimage in an unorientable material such as gelatin which is mounted upona base or support formed of a molecularly oriented plastic, thelight-polarizing image being formed in the plastic by the migration ofiodine from the gelatin.

Polymers characterized by having a multiplicity of hydroxy groupsextending off the main polymer chain and consisting of simple repetitivegrouping or units are generally useful as carriers for light-polarizingiodine images. Examples thereof are polyvinyl alcohol, polyhydroxyalkane, partially hydrolyzed polyvinyl acetals and polyvinyl alcoholesters, amylose and regenerated cellulose. In addition, other plasticssuch as suitably prepared polyamides or nylon-type plastics may beemployed for carrier materials.

Of the plastics herein named, polyvinyl alcohol is preferred.

To make long chain plastics of this character light-polarizing, theirmolecules must be substantially oriented. Orientation may be effected;

by stretching plastic sheet until the molecules thereof are sufiicientlyoriented. Aglternately, the surface molecules of one or both sides areplastic sheet may be oriented as by the application of linear,frictional forces thereto without.

4 orienting the molecules throughout the remainder of the sheet.

Of the carrier materials heretofore set forth, the invention, asintimated, intends to use orientable plastics in the formation of bothlight-polarizing images and nonpolarizing images. At the same time theinvention comprehends' the use of hydrophylic materials other thanorientable plastics, gelatin for one example, in the formation ofnonpolarizing images by novel methods which will presently be described.

A support is generally employed for the carrier or carriers of thephotographic films of the invention. The support may be a plasticmaterial although it may be glass and in certain instances paper whichmay have a metallic or other reflecting coating provided theron. In factany of the films to be later described may be superposed on alight-reflecting backing, at least after image formation, to provide areflection print.

Examples of suitable transparent support materials comprise a cellulosicplastic, for example cellulose esters such as cellulose acetate andcellulose nitrate or cellulose mixed esters such as cellulose acetatebutyrate or cellulose acetate propionate, Or a vinyl compound, such asthe vinyl acetate-chloride copolymers, or a suitable condensation typesuperpolymer, such as polyamide or nylon-type plastic. The various typesof cellulose esters such as cellulose nitrate and cellulose acetatebutyrate may be named as preferred transparent materials for thesupport. It is also to be understood that such materials as polyvinylalcohol and regenerated cellulose may be used to provide a support.

The different layers of the films with which the invention may bepracticed may be coated on or laminated to one another. When an orientedcarrier is employed, orientation may be effected after formation of thefilm, or in the case of the use of several carriers, they may beoriented before or after they are laminated.

The practice of the invention may be most simply understood withreference to the formation of light-polarizing images in a film Ill likethat forming the subject of illustration, modifications of the inventionbecoming apparent to those skilled in the art without further detailedillustration when considered in the light of the processing techniquesemployed for such film together with the explanation of suchmodifications.

The film l0 shown in the drawing comprises a base or support [2 on whichis supported a carrier or an image-bearing layer l4 formed of atransparent, hydrophilic, molecularly oriented polymeric plastic such aspolyvinyl alcohol, the oriented molecules of the layer beingschematically illustrated by the dash lines It. Sensitization of thecarrier M. of film It) provides a light sensitive element suitable forthe practice of the invention. Conventional silver halide formingsolutions may be imbibed in the carrier 14 to effect sensitization.Also, a solution of a plastic material of the character mentioned havingsilver halide dispersed therein may be cast into a carrier which maythen be subjected to processing such as that described to eifectorientation of the molecules of the plastic.

A silver image may be formed in the film l0 sensitized as just describedby suitable exposure and development in conventional manner. Thesilverimage is then transformed to an iodine image l8, the first; stepof the processing being to convert the silver image to an image formedof silver ferrocyanide as by bleaching or processing with a suitablealkali ferricyanide. The silver ferrocyanide image is in turntransformed to an image comprising a mixture of water-insol ublemetallicferrocyanides which do not include silver ferrocyanide. At least one ofthe ferrocyanides of the mixture can be converted to its correspondinghydroxide to provide an oxidizing agent having a sufiiciently highoxidation potential to release iodine from an acid solution of aniodide.

All of the metallic ferrocyanldes in the mixture may be transformed tohydroxides on suitable treatment. In such case at least one of thehydroxides which are formed lacks the-ability to serve as a potentialoxidizing agent capable of releasing iodine from an acidic iodidesolution. On the other hand certain ferrocyanides of a univalentmetallic element or a polyvalent metallic element in lower valent formare soluble in the reagent used for hydroxide conversion. For example,zinc ferrocyanide will be dissolved in a sodium hydroxide solution whichis a preferred treating solution for the image formed of the mixedferrocyanide's. However, as will presently appear, the removal of anyferrocyanide of a univalent or polyvalent metallic element in lowervalent form can take place at any stage of the processing subsequent toits formation without efiect on the final result to be attained.

Film it, after formation of the image comprising the oxidizing agent, istreated in an acid solution of an iodide, there-by releasing iodine fromthe iodide. As any hydroxides which form the image are soluble in acid,the hydroxide image is effectively removed and replaced by the releasediodine. As a result of this transformation the iodine is physicallylocated in the same portions of the film as the originally exposedsilver halide particles and forms the desired dichroic orlight-polarizing image 18 in the molecularly oriented carrier l2.

The principles underlying the invention may be understood when it isconsidered-that certain metallic salts may be employed to tone silverimages which have been bleached to silver ferrocyanide. Notable examplesare iron and copper salts employed to tone a silver image bleached forexample with potassium ferricyanide to an image of silver ferrocyanide.Toning of a silver ierrocyanide image with an iron or a copper salt inthe just-described manner will result in the formation of an image ofWater-insoluble iron or copper ferrocyanide.

Any metallic salt which forms an insoluble ferrocyanide upon reactionwith silver ferrocyam'de will serve in the processing of a silver imagebleached in this manner. If the processing agent is a salt of apolyvalent metallic element in higher yalent form, this higher valentmetallic ion will be capable of releasing iodine from an acid solutionof an iodide upon conversion of the ferrocyanide which is formed by suchmetallic salt to its corresponding hydroxide. On the other hand if theprocessing agent is a salt of a univalent element or of a polyvalentmetallic element in lower valent form, the metallic ion will beincapable of releasing iodine from acid solutions of iodides. Hydroxidesthus formed are soluble in acid solutions and hence are removed when thefilm is treated with an iodide-containing solution of acidic'character.

The processing solutions of this invention com- :prise a-mixture of atleast one salt of a poly.-

valent metallic element in higher valent form with atleast one salt of aunivalent metallic element other than silver or of a polyvalent metallicelement in a. lower valent form. The salts are selected to provideinsoluble metallic ferrocyanides upon toning of silver images bleachediodide. I

From this it follows that by controlling the proportions of the twotypes of metallic salts used to produce the image of mixedferrocyanides, the proportions of the ierrocyanides in the mixture canbe selected and the concentration of ions which can be made available torelease iodine from a given acid solution of an iodide can be controlledby the presence in the toning solution of the salt of aunivalentmetallic element or a polyvalent metallic element in lower valent form.As it is the oxidizing agent which causes the release of iodine, controlof the ion concentration.

of the polyvalent ions in higher valent form resulting from the quantityof oxidizing agent present will therefore determine the effectiveness ofthe oxidizing agent and the quantity of iodine releasedirom a giveniodide solution andhence the density of the iodine image.

Metallic salts which may be employed as the component providing ionswhich will subsequently secure the release of iodine comprise salts ofpolyvalent metallic elements in higher valent forms, such for example,as cupric or ferric salts. Metallic salts which may be employed as thecomponent for modifying the density of the final iodine image throughconversion of a portion of r the original silver image into a materialincapable of releasing iodine comprise salts of such univalent metallicelements as zinc or cadmium, and such polyvalent elements as nickel,uranium, vanadium, or cobalt in their lower valent forms.

In the process, as heretofore set forth, the transformation of silver tosilver ferrocyanide and the conversion of silver ferrocyanide to themixture of ferrocyanide salts of metallic elements have been indicatedas two distinct operations which may be carried out in separatesolutions. However, conversion of silver to the mixed metallicferrocyanides may be carried out more rapidly by effecting thetransformation in a single solu-- tion containing all the reactants,that is to say, the fe'rricyanide and the metallic salts. If thereactions are carried out in a single solution, precautions are taken toprevent direct reaction of the metallic salts with the ferricyanideadapted to form the silver ferrocyanide. This may be accomplished bytheaddition to the solution of a suitable composition comprising an organicsalt, such as ammonium oxalate, and an acid, such as oxalic acid. Othermaterials which may be used instead of ammonium oxalate for preventingundesired interaction between the components of the solution are, forexample, potassium oxalate, potassium citrate, ammonium citrate,potassiumsodium tartrate, ammoniumtartrate, sodium succinate and otherorganic salts. Instead of oxalic 3,Cldj. 0th8]l acids, such'ashydrochloric acid and weak .alkalies, for example, sodium carbonate,

potassium carbonate and ammonium carbonate, may be used.

, The novel features of this invention will more fully appear from adetailed descriptionof a typical exampleof the formation ofblack-and-white vectograph prints in which the photographic image is adichroic iodine image.

Silver halide-sensitized, molecularly oriented polyvinyl alcohol film isexposed to the subject to be reproduced, developed, stopped, and Washedin the usual manner, employing standard procedures therefor. 'Ihe filmwith at least one silver image therein is then treated with thefollowing solution, comprising bleaching and toning agents which aremixed in the order given below.

Cubic centimeters Ammonium oxalate, aqueous solution '70 Nickel nitrate,10% aqueous solution 10 Cupric sulfate, 10% aqueous solution 25Potassium ferricyanide, 10%, aqueous solution l2 Oxalic acid, 14%aqueous solution 4 A three minute treatment in the above solution isordinarily sufficient to bleach and process or tone the usual silverimage. However, the time may be varied depending upon the density of thesilver image obtained or on the qualities of the particular polyvinylalcohol-silver halide emulsion employed. The processed film is thenwashed for two minutes. As the next step, the cupric and nickelferrocyanides comprising the image are simultaneously converted to theirrespective hydroxides by treatment for example in an aqueous solutioncontaining 10 sodium 'thiocyanate, 10% sodium thiosulfate, and 1% sodiumhydroxide. This; solution also fixes silver halide remaining in thecarrier. Such practice is preferred from the standpoint of simplicityalthough if desired the carrier may be fixed by separate treatment andthen subjected to a sodium hydroxide solution to effect formation of theoxidizing agent. It is to be noted that in the example given there isemployed a control salt, that is nickel nitrate, which will not bedissolved in the formation of the oxidizing agent used to release iodinebut instead will be converted to its hydroxide. Treatment to form theoxidizing agent which will release iodine is continued for about 1minute after which the film is washed for about 3 minutes and is thentreated with an acid iodide solution to effect the formation of theiodine image.

A suitable iodide solution comprises:

Cubic centimeters n The product of this immersion contains an image iniodine. It is preferable to immerse the print in a final stabilizingbath, which bath contains boric acid to stabilize the dichroio image.The stabilizing bath mayalso contain sodium sulfate to prevent softeningof the sheet, potassium iodide to provide a control for the color of theimage and in some instances .a weak solution of sodium thiosulfate.

While the inventionhas been described in connection with the formationof a light-polarizing image or vectograph image in a film having asingle image carrier of molecularly oriented plastic material, it willbe understood that a film comprising two such layers, either on the sameside of the support or on opposite sides of the support,

.may have one or more pairs of such images formed therein. The imagesmay be of the same subject and may bear a predetermined relation to oneanother. For example, one may be the left-eye image and the other theright-eye image of a stereoscopic pair and the directions of molecularorientation of the plastic carriers for such images may be atsubstantially right angles to each other so that an observer of theiodine images formed in the carriers may view each pair of imagesstereoscopically with the aid of suitable light-polarizing viewers.

It will likewise be understood, to those skilled in the art, thatlight-polarizing images may be formed in a film similar to that of filmIll wherein the molecularly oriented, layer supports a layer of gelatinin which the iodine image is formed, the iodine migrating from thegelatin into the oriented plastic. Similarly, the practices heretoforeand subsequently described are adapted for the formation ofnonpolarizing images by the formation of iodine images in unorientablematerial, such as gelatin, or in orientable material, such as polyvinylalcohol, the molecules of which are in an unoriented condition.

The foregoing practices can be readily adapted for transforming thesilver image of a conventional photographic paper print to an iodineimage, which iodine image can then be transferred to a sheet comprisingat least a surfacelayer of molecularly oriented polyvinyl alcohol. In apreferred adaptation of the transfer process, the iodine is not releaseduntil the paper print is brought into intimate contact with thepolyvinyl alcohol surface.

This modification of the process comprises transforming the silver imagein the paper print to an image containing a hydroxide of a polyvalentmetal in higher valent form, as previously described above, and thenimmersing the print in a solution of an iodide which does not containan. acid. This brings the iodide into intimate contact with thehydroxide image but because of the absence of the acid does not releaseiodine. Acid for rendering the metallic hydroxide capable of elfectingthe release of the free iodine from the iodide solution is contained inthe oriented polyvinyl alcohol surface with which the print is thenbrought into surface contact. The iodine is thus simultaneously releasedand transferred to the polyvinyl alcohol surface. The acid, as forexample maleic acid, may be introduced into the polyvinyl alcoholsurface in any Well known manner, as by casting the polyvinyl alcohollayer or sheet with the acid contained in the casting solution. Whenthis transfer method of obtaining dichroic images is used, it isunnecessary to wash out the silver salts after the metallic ferrocyanideimage is formed.

In effecting the modified processing just described, the conventionalpaper print for a silver image formed therein is immersed in a treatingsolution such as that disclosed'for obtaining the image formed of themixture of water-insoluble ferrocyanides. Immersion in the solution forabout two minutes gives satisfactory results. The

paper print is then immersed in the sodium hy- 'droxide bath, eitherwith or without being first washed in sodium thiosulfate to removesilver salts. After this the print is dipped in a 10% potassium iodidesolution which contains no acid. Following this, the image carriercomprising a molecularly oriented sheet of a plastic material, such aspolyvinyl alcohol, and containing approximately 10% by weight of maleicacid, is brought 'into contact with the print whereby iodine is releasedand is transferred to the plastic carrier to give the desiredlight-polarizing image.

The film materials described maybe employed as out, roll or motionpicture film. They may be used as positive or negative forming materialsbut are especially adapted for positive printing purposes from preparednegatives. The products obtained by processin any of the photographicelements disclosed may be used as transparencies or they may be mountedupon a reflecting backing sheet, such as metallized paper and viewed asrefiection prints. It is to be kept in mind that the processingtechniques and carrier materials emloyed in the practice of theinvention may be utilized to provide an iodine image in each carrierlayer of a film and that such images may be nonpolarizing orlight-polarizing in character. Furthermore, the invention is especiallyadapted for the formation of images in the nature of stereoscopic pairswhereby to permit stereoscopic rendition or representation.

It should be pointed out that although the foregoing descriptionemphasizes the formation of iodine images which are duplicates ofphotographic silver images, the invention is in no way limited to theexclusive use of photographically obtained silver images.

It is also to be understood that the terms image in iodine or iodineimage as used in the claims refer to an image formed of iodine or itscomplexes.

From the foregoing it will be appreciated that the aims and objects ofthe invention have been accomplished in that novel processes have beenprovided whereby iodine images of predetermined density may be formed inphotographic carrier materials. Likewise, in attaining the aims andobjects of the invention, there has been provided a product which willpredeterminedly effect the release of iodine from an acid solution of aniodide whereby to form an iodine image of selected density.

Since certain changes in carryin out the above process, and certainmodifications in the product which embody the invention may be madeWithout departing from its scope, it is intended that all mattercontained in the above description, or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of forming an iodine image of predetermined density from asilver image which is supported'by a hydrophilic carrier, comprisingtreating the silver image to replace the silver thereof with a mixtureof water-insolublemetallic ierrocyanides other than silver andcomprising a predetermined quantity of at least one metallicferrocyanide which may be formed into an oxidizing agent having asuificient oxidization potential to release a predetermined quantity ofiodine from an acid solution of an iodide and at least one othermetallic ferrocyanide which is incapable of forming an oxidizing agentpossessing the ability to release iodine from an acid solution of aniodide by subjecting said silver image to the action of an alkaliferricyanide and a mixture of metallic salts formed of a predeterminedquantity of at least one metallic salt selected from the classconsisting of cupric salts and ferric salts and at least one othermetallic salt from the class consisting of zinc salts and cadmium saltsand of salts of nickel, uranium, vanadium and cobalt in which themetallic element is in lower valent form, transforming water-insolublemetallic ferrocyanide having-a metallic. element from said first-namedclass into the desired oxidizing agent for efiecting the release ofiodine by subjecting said mixture of water-insoluble metallicferrocyanides to the action of a base, and reacting said oxidizing agentso formed with said acid iodide solution whereby iodine in accordancewith the quantity of oxidizing agent present'is released and replacesthe oxidizing agent to form an iodine image of desired density.

2. A method of forming an iodine image of predetermined density from asilver image which is supported in a hydrophilic carrier, comprisingreplacing the silver image with a mixture of water-insoluble metallicferrocyanides other than silver ferrocyanide by treating the silverimage with an alkali ferricyanide and a mixture of metallic salts inpredetermined proportions to each other and in quantity sufficient toreact with substantially all silver ferrocyanide formed by the reactionbetween said alkali ferricyanide and said silver, said mixture ofmetallic salts comprising at least one metallic salt from the classconsisting of cupric salts and ferric salts whereby to provide at leastone water-insoluble metallic ferrocyanide capable of conversion to anoxidizing agent possessing the ability to effect the release of iodinefrom an acid solution of an iodide in accordance with the quantity ofoxidizing agent present and also at least one metallic salt from theclass consisting of zinc salts and cadmium salts and of salts of nickel,uranium, vanadium and cobalt in which the metallic element is in lowervalent form whereby to provide at least one water-insoluble metallicferrocyanide incapable of conversion to an oxidizing agent for efiectingthe release of iodine from acid iodide solution, converting metallicierrocyanide which has a metallic element from said first-named class toits correspondin hydroxide whereby to provide the desired oxidizingagentfor releasing iodine by treating the mixture of water-insolubleierrocyanides with a base and reacting said metallic hydroxide with saidacid iodide solution whereby iodine in accordance with the quantity ofsaid hydroxide present is released and replaces the hydroxide to form aniodine image of desired density.

3. In a method of forming an iodine image of predetermined density froma silver image in a hydrophilic carrier, the steps of treating saidsilver image with a solution comprising potassium ferricyanide, nickelnitrate and cupric sulfate in predetermined proportions to each otherwhereby to convert said silver to a mixture of cupric and .nickelferrocyanides, subjecting said mixed ferrocyanides to the action ofsodium hydroxide whereby to convert the mixed ferrocyanides to theircorrespondin hydroxides, and then causing the cupric hydroxide torelease iodine in proportion to its quantity present in the carrier bytreating the carrier with an acid solution of an iodide.

4. The method of forming a dichroic image of predetermined density,comprising forming an image in silver in a relatively water-insolublecarrier having at least a layer of a permeable, molecularly oriented,high polymer possessing an aiilnity for iodine and adapted to form adichroic sorption complex therewith, said layer supporting said silverimage, transforming the silver from said silver image to a mixture ofwater-insoluble metallic ferrocyanides other than silver and comprisinga predetermined quantity of at least one metallic ferrocyanide which maybe formed into an oxidizing agent having 'a -sufficient oxidizationpotential to release a--predeter mined quantity of iodine from an acidsolution of an iodide and at least one other metallic ferrocyanide whichis incapable of forming an oxidizing agent possessing the ability torelease iodine from an acid solution of an iodide by subjecting saidsilver image to the action of an alkali ferrocyanide and a mixture ofmetallic salts formed of a predetermined quantity of at least onemetallic salt'selected from the class consisting of cupric salts andferric salts and at least one other metallic salt from the classconsisting of zinc salts and cadmium salts and of salts of nickel,uranium and vanadium and cobalt in which the metallic element is inlower valent form, transforming Water-insoluble metallic ferrocyanidehaving a metallic element from said first-named class into the desiredoxidizing agent for effecting the release of iodine by subjecting saidmixture of water-insoluble metallic ferrocyanides to the action of abase, and reacting said oxidizin agent so formed with said acid iodidesolution whereby iodine in accordance with the quantity of oxidizingagent present is released and replaces the oxidizing agent to form aniodine image of desired density.

5. The method of producing a light-polarizing image of predetermineddensity in iodine, which comprises forming the image in silver in apermeable, relatively water-insoluble carrier, treating the silver imageto replace the silver thereof with a mixture of water-insoluble metallicferrocyahides other than silver and comprising a predetermined quantityof at least one metallic ferrocyanide which may be formed into anoxidizing agent having a suflicient oxidization potential to release apredetermined quantity of iodine from an acid solution of, an iodide andat least one other metallic ferrocyanide which is incapable of formingan oxidizing agent possessing the ability to release iodine froman acidsolution of an iodide by subjecting said silver image to the action ofan alkali ferrocyanide and a mixture of metallic salts formed of apredetermined quantity of at least one metallic salt selected from theclass consisting of cupric salts and ferric salts and at least one othermetallic salt from the class consisting of zinc salts and cadmium saltsand of salts of nickel, uranium, vanadium and cobalt 7 iodine isreleased in accordance with the quantity of oxidizing agent present andreplaces the oxidizing agent to form an image in iodine of predetermineddensity, and bringing into face-toface contact with said carrier atransparent, molecularly oriented high polymer adapted to form adichroic sorption complex with iodine and having a greater afiinity foriodine than said carrier. HELEN P. HUSEK.

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